The processing of industrial gases, such as normally gaseous hydrocarbons and hydrogen, typically requires the separation of both acid-gas constituents and water prior to subsequent processing or distribution.
Acid gas constituents, such as CO.sub.2 and H.sub.2 S, are quite commonly removed from industrial gases by counter-current absorption with an aqueous solution of an acid gas absorbent. The acid gas absorbent solution is typically regenerated by distillation to remove the absorbed acid gases and recycled to the absorption step. The acid free (sweetened) product gas is subsequently dried in a separate operation.
Hygroscopic liquids are widely used for drying numerous industrial gases, by passing the comparatively dry, liquid hygroscopic desiccant through a column in a direction counter to the flow of the gas. Dry gas is usually removed from the top of the column, while the rich liquid desiccant, containing the absorbed water, is removed from the bottom of the column and sent to a regenerating unit where the rich desiccant is heated to remove as much absorbed water as possible without excessively decomposing the desiccant. The so regenerated liquid desiccant is then recycled to the gas drying column.
The hygroscopic nature of the desiccants serve well for absorption of water, however, there are problems in their regeneration. With heat alone, a temperature approaching the boiling point of the desiccant, which frequently exceeds the decomposition range of the liquid desiccant, is needed to reduce the moisture content to a level satisfactory for the intended desiccant application. More serious, however, are the effects of direct contact of the desiccant with hot surfaces of a heat exchanger. It is known that excessive heat fluxes on such surfaces contribute greatly to break-down of the desiccants.
Previous attempts at solving the problems have not been entirely satisfactory.
In one process, shown in U.S. Pat. No. 3,105,748, an aliquot of dried natural gas is heated to 325.degree.-365.degree. F in a gas-fired heater. This temperature is slightly below the decomposition point of a glycol moisture absorbent. The heated gas is passed through a hot glycol maintained at about the temperature indicated above. The gas used for stripping moisture from the glycol is vented or flared. Thus, this process is wasteful of gas and requires careful control to avoid loss of glycol through decomposition by contact with hot surfaces in the reboiler. In addition, the vented or flared gas contributes to air pollution.
In another procedure, described in U.S. Pat. No. 3,349,544, an azeotroping agent is introduced below the surface of a liquid desiccant in a heated regeneration zone, wile maintaining the temperature in the regenerating zone above the vaporization temperature of the azeotroping agent. The azeotropic mixture is condensed, water and azeotroping agent are separated and the azeotroping agent is recycled. In this process the moisture content of the desiccant cannot be reduced below that consistent with water contained in the azeotroping agent. It also exposes the desiccant to hot surfaces.
In a process described in U.S. Pat. No. 3,471,370, naptha and a glycol-amine desiccant-acid gas absorbent are fed to a reboiler, situated externally of a stripping column, into which a moist glycol-amine is fed. The temperature of the reboiler is maintained at 300.degree.-400.degree. F. In the reboiler, all the naphtha, except that which may remain in solution in the absorbent at the high temperature, is flashed and the vapors are passed through the moisture stripping zone to dry the glycol-amine mixture to a moisture content of about 1.6 to 1.8 percent. In this procedure, the moisture content of the desiccant cannot be reduced below that consistent with the water content of the naphta. It also has the shortcoming of exposure of the desiccant to hot surfaces.
A further shortcoming of the prior processes is that very high boiling desiccants, such as tetraethylene glycol, higher boiling glycols or other polyols or alkanolamines cannot be employed in continuous gas drying processes, because of excessive decomposition at temperatures necessary to liberate water.